The present invention relates generally to a frequency converter circuit comprising a high-frequency oscillator, a buffer amplifier and a mixer for converting a high-frequency signal into an intermediate-frequency signal.
In a satellite broadcast receiving system, a signal of 12 GHz band is received by a parabolic antenna, and down-converted into a signal of 1 GHz band by an outdoor unit. This signal is further converted into a 400 MHz signal by an indoor unit. The present invention provides a frequency converter circuit suitably used for applications of converting the above-mentioned signal of 1 GHz band into a signal of 400 MHz.
A typical conventional frequency converter circuit for converting a high frequency into an intermediate frequency is disclosed in Japanese Patent Unexamined Publication No. 179005/83.
Such a frequency converter circuit comprises a collector-grounded transistor oscillator and a mixer including two diodes, in which an oscillation frequency is applied from an inductor of a resonance circuit of the oscillator by inductive coupling to the mixer circuit.
Generally, in a frequency converter circuit, the oscillation power applied to a mixer is recovered by inductive coupling as described above, and if a large power sufficient to secure satisfactory mixer operation is to be obtained, a buffer transistor is used with a closer inductive coupling. The output of this inductive coupling changes the resonance impedance, thus easily causing such an abnormal oscillation as an oscillation skip or oscillation stop. Further, if the oscillation circuit is variable over a broad band to make possible broad-band receiving, the resonance characteristic of the inductive coupling circuit becomes conspicuous, thereby causing a large frequency deviation of the oscillation power applied to the mixer (that is, a large variation of the oscillation power with frequency), resulting in a large frequency deviation of the frequency conversion characteristic of the mixer.
Since the conversion loss of the mixer circuit is great, on the other hand, it is necessary to arrange amplifiers in multiple stages after the mixer.
Furthermore, the buffer transistor requires large power for driving the mixer with large oscillation power, thereby leading to the disadvantage of a large leakage of the oscillation power of the mixer to the signal input terminal thereof.